fix(netcmp): correct signal handler type for K&R declaration

netcmp.c:55 declares oldinthandler with empty parameter list:
    void (*oldinthandler)() = SIG_DFL;

In K&R / pre-C23, this means 'function with unspecified parameters'.
GCC 14+ infers void(*)(void), which is incompatible with signal(2)'s
expected void(*)(int) handler. The signal(SIGINT, oldinthandler) calls
at lines 8777 and 8784 then fail with -Wincompatible-pointer-types
(now a default error in GCC 14+).

This 1-line fix matches the actual usage as a SIGINT handler with int
signum parameter, and restores tclnetgen.so build on Fedora 41+ /
Debian 13+ / Ubuntu 24.04+ (any system with GCC 14+).

Tested: tclnetgen.so now builds successfully and 'netgen -batch lvs'
mode works again.

base/netcmp.c

@@ -52,7 +52,7 @@ the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA. */
 
 #ifdef TCL_NETGEN
 int InterruptPending = 0;
-void (*oldinthandler)() = SIG_DFL;
+void (*oldinthandler)(int) = SIG_DFL;
 extern Tcl_Interp *netgeninterp;
 extern int check_interrupt();
 #endif